Basically, it wasn't possible to fit all 7 LTE bands that apple wanted to support into one device, so they separated the US/Canada GSM iPhone into a separate phone - A1428 (LTE bands 4 and 17).

The A1429 comes in two varieties, GSM and CDMA (although the CDMA model appears to be a complete superset of all frequencies of the GSM model).

The two additional bands that the CDMA model support (13 and 25) are only used by Verizon and Sprint in the US.

So the 'international' GSM model only supports bands 1, 3, and 5 because it's not possible to use the phone with Verzion or Sprint networks (these companies do not use a sim card, and can only be activated on CDMA if you purchase the phone through Sprint or Verizon.

It makes me think the A1429 GSM and CDMA models are actually identical hardware wise (similar to the 4S).

A GSM purchased 4S can never be activated on CDMA, as it was not purchased through a CDMA carrier. So this time, Apple have marketed the GSM A1429 iPhone 5 without CDMA capability because it can never be utilised.

Edit: I'm now 95% percent certain that the GSM A1429 and CDMA A1429 are identical hardware wise. The GSM version removes CDMA references (including the two LTE bands the CDMA operators use), as you would not ever utilise the CDMA part of a GSM A1429 phone.

Edit 2: So to answer drquack32's question, there are two models because it wasn't possible to accommodate all LTE bands into one phone. Buy the GSM A1429 when it releases here, don't feel like you're missing out on those two extra LTE bands because no GSM carrier currently supports them anyway. I would imagine that if a GSM carrier started using bands 13 and 25, the GSM A1429 would be compatible, provided Apple haven't blocked the bands for use on GSM.

HotDogBreath: Vodafone seem to only offer 3G speeds in major centers (correct me if I'm wrong here) so is the LTE roll out also going to be limited to the same coverage?

Vodafone have offered nationwide 3G coverage for around 4 years now. There are also plenty of areas that have 3G coverage that don't have GSM coverage.

One would expect LTE coverage to be limited initially from all carriers, since the 1800MHz spectrum they have at present is fine for urban areas, but not necessarily ideal for rural areas. Since the 700MHz spectrum isn't going to be auctioned off until later this year, and right now not a single hardware vendor is delivering LTE gear for this band, and no device manufacturer building devices then we're probably a good 2 years away from seeing this band in use.